1. Field
The following description relates to a bulk acoustic wave resonator and a filter.
2. Description of Related Art
Due to limitations on frequency resources for mobile communications systems, such as cellular phone networks, an amount of a usable frequency is very limited and, as a result, very costly for an end user.
Transmission frequencies and reception frequencies having a predetermined amount of band gap therebetween have been used in order to avoid interference between transmitted and received signals.
As described above, because it is very costly to obtain usable frequencies, it is important to ensure that the usable frequencies are effectively and efficiently allocated and used. As a result, there is a need to reduce band gaps between transmission frequencies and reception frequencies in order to efficiently use frequency resources.
In a general wireless communications system, band gaps are reduced by implementing a duplexer separating a transmitted signal and a received signal from each other, but high quality factor (QF) is required.
In addition, the wireless communications needs to be stable regardless of ambient temperature. A temperature coefficient of frequency (TCF) is a temperature of a wireless signal filter. In order to implement stable wireless communications, a frequency change depending on a TCF is very important. An apparatus and method that allows a TCF value of the wireless signal filter to be close to ‘0’ is required. Furthermore, the wireless signal filter uses a bulk acoustic wave resonator. A temperature compensation layer may be added to implement a low TCF value, but a noise peak may occur in an unwanted frequency band by the temperature compensation layer.